Horizontally opposed compressors are conventionally designed with the opposed cylinders offset. In 2 and 4 throw cylinder compressors an unbalanced dynamic couple force is inherent due to the offset of the opposed reciprocating mass. In 6 throw cylinder compressors the unbalanced dynamic couple forces can be substantially balanced out with proper phasing of the cylinders. U.S. Pat. No. 3,474,768 describes a diametrically opposed 4 cylinder engine with two concentric shafts that only rotate 180.degree., the 180.degree. rotation is converted to 360.degree. rotation by a connecting rod transferring the 180.degree. rotation of each of the concentric shafts to a crankshaft.